The flexible metal pipes may be produced in conventional fashion by the coiling of a profiled interlocking strip (for example as per FR 2 555 920) or of a wire with interconnected helical turns (for example as per FR 2 650 652) or by any other process that gives the pipe good flexibility.
Flexible tubular conduits generally incorporate a flexible metal pipe serving as the inner frame which is formed by a helically coiled, profiled metal strip, for instance with inter-locking turns, which interlocking strip-coil frame is covered with an impervious polymer sheath and the entire assembly is covered with reinforcing layers to withstand pressure as well as the underwater environment. Such flexible conduits are described for instance in patents FR 2.619.193 and in "Recommended Practice for Flexible Pipe--API Recommended Practice 17 B (RP 17 B) First Edition Jun. 1, 1988".
Bending of the flexible metal pipes is made possible by providing spaces between the helical turns. The interconnection between the turns is never impervious to liquids or to gas. Therefore, an impermeable polymer sheath is fitted over the metal pipe. One may use for instance vulcanized rubber or, for conduits having greater mechanical strength, a thermoplastic polymer offering the required mechanical properties, for example polyethylene, for moving water or degassed crude oil in the extraction of underwater deposits.
What is most wanted, however, is to find a polymer material which offers three qualities: Low permeability to liquids and/or gas, resistance to a wide range of operating temperatures (both mechanical resistance and chemical insensitivity to high temperatures), and easy industrial implementation. Certain semicrystalline polymers possess all of these qualities, with the more crystalline types among them being of particular interest due to their low permeability. On the other hand, the higher the rate of crystallinity of a polymer, the higher its rate of physical stress as it passes from the molten state to its crystallized solid state. If this shrinkage is prevented as in the case of a sheath extruded around a metal pipe, residual stress is produced especially in the form of tension within the polymer, weakening the sheath's shock resistance and flexibility.
Moreover, when the polymer sheath is extruded onto the metal pipe, the polymer enters into the spaces between the helices, thus reducing the degree of flexible movement of the pipe. Depending on the required properties and the intended use of the flexible pipe, such interstitial penetration of the polymer is acceptable in many cases. For certain applications this penetration effect is even sought intentionally (FR 2 268 614). However, given that high-resistance flexible conduits are envisioned for heavy-duty operating conditions, it has been found that the penetration of the polymer in the spaces between the helices can have a negative effect on the performance of the sheath. In particular, studies have revealed initial fissures which can lead to progressive ruptures and to leaks both locally and at the perimeter of the raised section of the sheath as a function of the degree of polymer penetration between the helical turns.
For flexible pipes used in oil or gas extraction where the sheath material must also stand up to live crude without blistering or inflating, the metal pipes can be sheathed with polyamide-11 (PA-11) or, for more demanding operating conditions, with a fluorinated polymer, in particular polyvinylidene fluoride (PVDF). Polyvinylidene fluoride, by virtue of its crystallinity, chemical near-insensitivity and imperviousness to liquids and gas as well as its resistance to a temperature on the order of 105.degree. C. over many years, is the material of choice for the sheathing of flexible metal pipes, yet its rigidity does not permit such use.
To overcome this drawback, the PVDF may be plasticized. However, experience shows that the plasticizers migrate out of the polymer, causing the latter to return to its original rigidity over a period of time depending again on the temperature of the liquids flowing through the pipe. One can also use plasticized PA-11 to produce a leak-proof polymer sheath for flexible metal pipes. As an alternative to the modification of an excessively rigid polymer by the application or admixture of a plasticizer, another known approach has been to copolymerize a predominant part of the monomer corresponding to at least one other comonomer.
Nevertheless, the polymer sheaths that can be produced by known methodology have limitations in their possible uses, the limitations being dependent upon performance requirements, especially when the pipe is to carry live crude oil under high pressure and/or at high temperatures. On the one hand, plasticized polymers are affected by the migration of the plasticizers and, in spite of the plasticizing, they also involve the risk of a weakening in the areas between the helices when subjected to severe operating conditions. On the other hand, certain extra high-performance polymers whose use would be of interest with no or relatively little plasticizing remain practically ineligible due to their excessive rigidity.